The study, accepted for publication in the journal Food Control, reports that the extremely toothy fish, with saw snouts, wind up skinned, filleted and displayed in markets. Consumers have no idea what they’re buying.

"The proper identification of fish products, like fillets, is a worldwide concern, and several studies have focused on unreported or misidentified fishing issues," wrote co-author Iracilda Sampaio of the Universidade Federal do Pará and colleagues, after identifying some "fresh or salted fillets" in markets as being flesh from largetooth sawfish.

First, Sampaio and her team sequenced two genes from largetooth sawfish. They then performed genetic analysis of 44 samples of fish sold at markets in northern Brazil. The samples were sold under the generic label of “shark.” Twenty-three out of the 44 samples had genes that matched those of largetooth sawfish.

Fishing and trade of this species have been illegal for some time in Brazil, as well as in the United States and certain other countries. Nevertheless, as Sampaio pointed out, they are often "caught as bycatch in fisheries targeting commercially important species." These include swordfish, catfish, tuna and other fish.

George Burgess, director of the Florida Program for Shark Research at the Florida Museum of Natural History, told Discovery News that the problem happens in the United States too.

"The snout and teeth of sawfish are easily caught in nets," he explained. "Sawfish also spend most of their time right on our doorstep, visible in shallow, brackish water, so they are more vulnerable."

A typical day for a sawtooth might begin by hunting for other, smaller fish and crustaceans. They often do this near river mouths and large bays, preferring partially enclosed waters. Once they make contact with a fishermen’s net, their long saws and teeth become entangled, giving them little chance for escape.

"They also readily take baited hooks," said Burgess, reminding that line fishermen can catch sawfish too.

Once caught, they wind up in markets, say researchers, or mounted.

"Largetooth sawfish can grow up to 25 feet long and are globally endangered, but they are often killed, with their snouts winding up on a wall somewhere," Burgess said, adding that out of all of the world's sharks and rays, he is most concerned now about sawfish.

"The smalltooth sawfish is also endangered," he continued. "The population is barely hanging on now in South Florida, and it would take 100 years or more for it to fully recover."

"Giant sawfish rostrums" are not hard to find on eBay and other online sites, but for each sawfish, eBay requires that sellers "authenticate that it is antique" and was killed before the endangered species entry.

Some cultures believe that tea made from the saws helps to treat asthma, so it's sold dried in folk remedies. Sawfish fins are highly prized in the Asian shark-fin soup market. Teeth become artificial spurs in the brutal cockfighting industry.

Given pressure from this human demand, along with habitat loss and pollution, the outlook for sawtooths appears bleak. The situation is extremely serious from "economic (fraud) and environmental (stock management) viewpoints," according to the researchers. They at least hope that DNA testing will provide one effective tool in the fight to save sawtooths.